1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to a dual plate organic electroluminescent device that includes a first substrate having a thin film transistor array unit and a second substrate having an organic electroluminescent unit, and a method of fabricating the same.
2. Discussion of the Related Art
Generally, an organic electroluminescent device (ELD) emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Unlike the case for a liquid crystal display (LCD) device, an additional light source is not necessary for the organic ELD to emit light because the transition of the exciton between states causes light to be emitted in the organic ELD arrangement. Accordingly, the size and weight of the organic ELD can be reduced. The organic ELD has other desirable characteristics, such as a low power consumption, superior brightness and fast response time. Because of these advantageous characteristics, the organic ELD is regarded as a promising candidate for various next-generation consumer electronic applications, such as cellular phones, car navigation systems (CNS), personal digital assistants (PDA), camcorders and palmtop computers. Moreover, because fabricating an organic ELD is a relatively simple process with few processing steps, it is much cheaper to produce an organic ELD than an LCD device.
Two different types of organic ELDs exist: passive matrix and active matrix. Even though passive matrix organic ELDs have a simple structure and are formed by a simple fabricating process, passive matrix organic ELDs require a relatively high amount of power to operate. In addition, the display size of passive matrix organic ELDs is limited by their structure. Furthermore, as the number of conductive lines increases, the aperture ratio of passive matrix organic ELDs will decrease. On the other hand, active matrix organic ELDs contrast with passive matrix organic ELDs in that the active matrix organic ELDs have a high emission efficiency and can produce high-quality images for a large display with relatively low power consumption.
FIG. 1 is a schematic cross-sectional view of an organic electroluminescent device according to the related art.
As shown in FIG. 1, an organic electroluminescent device (ELD) 10 includes first and second substrates 12 and 28 facing each other and spaced apart from each other. An array layer 14 including a thin film transistor (TFT) “T” is formed on an inner surface of the first substrate 12. A first electrode 16, an organic electroluminescent (EL) layer 18, and a second electrode 20 are sequentially formed on the array layer 14. The organic EL layer 18 may separately display red, green, and blue colors for each pixel region “P.” Generally, separate organic materials are used to emit light of each color for the organic EL layer 18 in each pixel region “P.” The organic ELD 10 is encapsulated by attaching the first substrate 12 and a second substrate 28, which includes a moisture absorbent desiccant 22, with a sealant 26. The moisture absorbent desiccant 22 eliminates moisture and oxygen that may penetrate into a capsule of the organic EL layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the moisture absorbent desiccant 22 and the filled moisture absorbent desiccant 22 is fixed by a holding element 25.
FIG. 2 is a schematic plane view showing an array layer of an organic electroluminescent device according to the related art.
As shown in FIG. 2, an array layer of an organic electroluminescent device (ELD) includes a switching element “Ts,” a driving element “TD” and a storage capacitor “CST.” The switching element “TS” and the driving element “TD” may include a combination of at least one thin film transistor (TFT). A transparent insulating substrate 12 on which the array layer is formed may be made of glass or plastic. A gate line 32 and a data line 34 crossing each other are formed on the substrate 12. A pixel region “P” is defined by the gate line 32 and the data line 34. An insulating layer (not shown) is interposed between the gate line 32 and the data line 34. A power line 35 parallel to and spaced apart from the data line 34 crosses the gate line 32.
The switching element “TS” in FIG. 2 is a thin film transistor that includes a switching gate electrode 36, a switching active layer 40, a switching source electrode 46, and a switching drain electrode 50. Similarly, the driving element “TD” in FIG. 2 is a thin film transistor that includes a driving gate electrode 38, a driving active layer 42, a driving source electrode 48, and a driving drain electrode 52. The switching gate electrode 36 is connected to the gate line 32 and the switching source electrode 46 is connected to the data line 34. The switching drain electrode 50 is connected to the driving gate electrode 38 through a first contact hole 54. The driving source electrode 48 is connected to the power line 35 through a second contact hole 56. Moreover, the driving drain electrode 52 is connected to a first electrode 16 at the pixel region “P.” The power line 35 overlaps a first capacitor electrode 15 with the insulating layer interposed therebetween to form the storage capacitor “CST.”
FIG. 3 is a schematic plane view showing an organic electroluminescent device according to an arrangement of the related art.
As shown in FIG. 3, a substrate 12 includes a data pad region “E” at a first side and first and second gate pad regions “F1” and “F2” at second and third sides adjacent to the first side. A common electrode 39 is formed at a fourth side facing the first side and adjacent to the second and third sides of the substrate 12. A common voltage is applied to a second electrode 20 through the common electrode 39 to maintain an electrical potential of the second electrode 20.
FIG. 4A is a schematic cross-sectional view taken along a line “IVa—IVa” of FIG. 2 and FIG. 4B is a schematic cross-sectional view taken along a line “IVb—IVb” of FIG. 3.
In FIGS. 4A and 4B, a driving thin film transistor (TFT) “TD” including a driving active layer 42, a driving gate electrode 38, a driving source electrode 48, and driving drain electrode 52 is formed on a substrate 12. An insulating layer 57 is formed on the driving TFT “TD” and a first electrode 16 connected to the driving drain electrode 52 is formed on the insulating layer 57. An organic electroluminescent (EL) layer 18 for emitting light of a specific color is formed on the first electrode 16, and a second electrode 20 is formed on the organic EL layer 18. The first and second electrodes 16 and 20 together with the organic EL layer 18 interposed therebetween constitute an organic electroluminescent (EL) diode “DEL”. A storage capacitor “CST” situated to be electrically parallel with the driving TFT “TD” includes first and second capacitor electrodes 15 and 35a. A portion of a power line 35 (of FIG. 2) overlapping the first capacitor electrode 15 is used as the second capacitor electrode 35a. The second capacitor electrode 35a is connected to the driving source electrode 56. The second electrode 20 is formed over an entire surface of the substrate 12 including the driving TFT “TD,” the storage capacitor “CST” and the organic EL layer 18.
A common electrode 39 through which a common voltage is applied to the second electrode 20 is formed at a peripheral portion of the substrate 12. The common electrode 39 is simultaneously formed with the switching gate electrode 36 (of FIG. 2) and the driving gate electrode 38. A plurality of insulating layers on the common electrode 39 include first and second common contact holes 50 and 52, which expose the common electrode 39. The second electrode 20 is connected to the common electrode 39 through the first common contact hole 50. An external circuit (not shown) is connected to the common electrode 39 through the second common contact hole 52 to supply the common voltage.
However, when an array layer of TFTs and organic EL diodes are formed together on one substrate, a production yield of an organic ELD is determined by a multiplication of TFT's yield and organic EL layer's yield. Since organic EL layer's yield is relatively low, the production yield of an ELD is limited by the organic EL layer's yield. For example, even when a TFT is well fabricated, an organic ELD can be judged as being bad due to defects of an organic EL layer using a thin film of about 1000 Å thickness. This limitation causes loss of materials and rise in production cost.
Organic ELDs are classified as being a bottom emission type or a top emission type according to a transparency of the first and second electrodes and of the organic EL diode. The bottom emission type ELDs are advantageous for their high image stability and variable fabrication processing due to encapsulation. However, the bottom emission type organic ELDs are not adequate for implementation in devices that require high resolution due to the limitations of the increased aperture ratio in that type of organic ELDs. On the other hand, since top emission type organic ELDs emit light in a direction upward of the substrate, the light can be emitted without influencing the array layer that is positioned under the organic EL layer. Accordingly, the overall design of the array layer including TFTs may be simplified. In addition, the aperture ratio can be increased, thereby increasing the operational life span of the organic ELD. However, since a cathode is commonly formed over the organic EL layer in the top emission type organic ELDs, material selection and light transmittance are limited such that light transmission efficiency is lowered. If a thin film type passivation layer is formed to prevent a reduction of the light transmittance, the thin film type passivation layer may fail to prevent infiltration of exterior air into the device.